Spread-spectrum communications, microwave signal processing, phased array antenna systems, and digital RF memory are but a few of the applications where extremely broadband signal processing, in some cases several gigahertz, is required. High resolution state-of-the-art (commercial off-the-shelf Analog to Digital Conversion Systems ADCs) have a conversion rate of less than a hundred megahertz. Consequently, the conversion rate of an ADC presents a major bottleneck to the digital processing speed. To accommodate the need for broadband data acquisition and processing, present systems sometimes employ sub-band coding techniques. Using these techniques, the analog signal to be digitized is first passed through a bank of band-pass filters. The bandwidth-reduced output of each of these filters is then separately digitized and coded so that the digital representation retains all the information contained in the original analog signal. Although effective, this process is computationally intensive and thus unsuitable for real time high-speed applications.
Though the advantages of optical processing for broadband signals are well known, such processing is often analog, not digital, in nature and usually application specific. However, optical techniques for realizing analog to digital conversion have been developed, as illustrated by U.S. Pat. No. 4,571,576 to Olsson et al. This patent discloses a high speed analog to digital converter that uses a frequency tunable laser that emits radiation at a frequency which is determined by an input voltage. The device uses a grating which disperses and directs particular wavelengths of the radiation to an array of photo-detectors. Each photo-detector receives and detects radiation of one wavelength from the grating and converts the radiation into a digitized signal.
Also U.S. Pat. No. 4,964,276 to Rastegar discloses the use of an optical reflective grating in an optical analog to digital converter.
Many prior optical techniques for implementing analog to digital conversion have been hybrid electrical/optical systems, such as systems using a pulsed laser as a means to rapidly sample an analog signal. The samples are then M-decimated and routed into M optical channels. Each channel is photo-detected and the output is directed into one of a bank of low speed electronic analog to digital converters which function in parallel. The binary outputs from each of the ADC""s is time multiplexed to reassemble the binary word corresponding to the broadband input. Though a laser and modulator are used to sample the analog signal, the ADC is otherwise of the traditional electronic type.
It is a primary object of the present invention to provide a novel and improved analog to digital converter and method which is completely optical and capable of achieving the rapid conversion rate necessary for the digital processing of extremely broadband signals.
Another object of the present invention is to provide a novel and improved analog to digital conversion method using all-optical signal processing providing conversion rates several orders of magnitude faster than the fastest electronic processor.
Yet another object of the present invention is to provide a photonic wide-band analog to digital converter with a minimum number of active components amenable to integration in the form of a photonic integrated circuit.
A further object of the present invention is to provide a photonic wide-band analog to digital converter which is lightweight, compact, operates on a low power budget and is fully compatible with optical fiber-based data distribution links. For an N-bit analog to digital converter, one laser and N photo-detectors are the only active components required.
A still further object of the present invention is to provide a method for converting an analog electrical signal to a digital electrical signal by means of optical signal processing. The analog electrical signal is converted by a tunable laser to an optical signal which is a function of the amplitude of the analog signal. This optical signal is converted in an optical processor to optical bit signals which form an N bit binary word, and these optical bit signals are then converted to electrical bit signals.
These and other objects of the present invention are achieved by providing an analog electrical signal to be digitized to a tunable laser which provides an optical output signal having an optical wavelength which is a function of the amplitude of the analog electrical signal. An optical processor is connected to receive the optical output signal and to convert it to an N bit binary word. This processor includes N separate optical bit legs which each include optical filters for providing an optical output bit signal indicative of one bit of the N bit binary word. Photo-detectors connected to the optical processor operate to convert the optical bit signals to electrical bit signals.